1. Field of the Invention
The invention relates to a resonant converter, comprising a first and a second input terminal in order to connect a voltage source, wherein the first end of a first switch is connected to the first input terminal, wherein the second end of the first switch is connected to the first end of a second switch, wherein the second end of the second switch is connected to the second input terminal, wherein a first end of a resonant circuit comprising at least one resonant inductance, at least one resonant capacitor, and at least a primary winding of a transformer is connected to the second end of the first switch and to the first end of the second switch.
2. Description of the Related Art
Resonant converters, in particular LLC resonant converters, have a growing popularity due to their high efficiency, low level of EMI emissions, and ability to achieve high power density. Further advantages are small size, light weight and high efficiency.
Typical topologies of LLC resonant converters comprise half-bridge and full-bridge topologies. In a resonant tank or network, resonant inductors and resonant capacitors are arranged, wherein both serial and parallel topologies are known. In zero-voltage-switching (ZVS) or zero-current-switching (ZCS) topologies, high-frequency switching as well as minimized switching losses are of interest. LLC resonant converters operating at high frequency can improve the efficiency and reduce the size of the devices.
A requirement for many applications is a sufficient hold up time of the converter, such that in case of a power failure, full power output is maintained for a sufficiently long period, e.g. for about 20 ms to 50 ms.
Moreover, the converter has to be capable to operate safely and without damages in short circuit or overload conditions, i.e. in case of a short circuit of the output or higher than specified power consumption of a consumer connected to the output. Accordingly, the components of the converter, particularly the resonant capacitor, have to be protected from over voltage conditions which may occur in such overload or other conditions.
In a known half-bridge topology, a voltage source is connected to a pair of switches connected in series and a pair of resonant capacitors connected in series. A primary winding of a transformer is connected between the switches and the resonant capacitors. Such a topology is widely used for various applications.
In “Topology Investigation for Front End DC/DC Power Conversion for Distributed Power System”, Bo Yang, 2003, Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University, it is proposed to connect diodes in parallel to the resonant capacitors of the known half-bridge topology, in order to limit the resonant capacitor voltage and resonant current during overload or short circuit conditions of the output.
The output power region at which clamping of the resonant capacitor voltage occurs depends on the dimensioning of the resonant circuit elements. For reasonable design, the clamping should go into effect near above the rated output power. On the one hand this ensures best resonant current limiting and on the other hand this is the most efficient design considering partial load conditions. The higher the resonant capacitor voltage is set by design, the higher is the part load efficiency of the converter.
However, the described clamping method is disadvantageous for the boost characteristic of the converter used for hold up functionality. Because during hold up condition the bulk voltage is being decreased, the region of output power at which clamping of the resonant capacitor voltage occurs is expanded, which results in reduced hold up time in case of high output power.